In Time Division Multiple Access (TDMA) transmission, the satellite transponder is used or accessed by a number of earth stations in an ordered time sequence. The transmissions are timed such that the Phase Shift Keyed (PSK) modulated RF carriers of the same frequency arrive at the satellite without overlap. The primary advantages of the TDMA method for commercial satellite communications are the efficient utilization of the satellite power and frequency spectrum and the flexibility provided in reconfiguring the TDMA burst and traffic patterns.
In general, a TDMA terminal must perform two basic functions. First, the terminal must form a burst from a number of continuous stream of data and transmit this data with a preamble at a specified time such that the transmitter burst does not interfere with any other burst transmission. Second, the terminal must receive burst transmissions intended for the terminal, recover the continuous data streams from the received bursts and route the data to the correct terrestrial interface module (TIM).
Accordingly, in the transmit mode of operation, the terminal must typically send read address signals to TIM ports, receive corresponding data from the addressed ports and prepare the data for TDMA transmission in a burst format. Depending on the desired burst size, the data is divided into blocks, and to each block of data is added a preamble containing carrier and bit timing acquisition symbols, origin code information, range data, order wire information, etc. The burst formed by the data block and the preamble is then scrambled and sent to the modulation equipment for modulation onto a carrier and subsequent transmission.
In the receive mode of operation, the demodulated bursts are received from the modulation and demodulation equipment (MODEM) and essentially the reverse processing is accomplished so that continuous data and corresponding write address signals can be sent to the proper TIM ports.
As described above, modulated carriers of the same frequency must arrive at the satellite transponder without overlap. Accordingly, participating stations in a TDMA system may be allocated certain burst positions within a TDMA frame, and each station must time its transmissions such that its burst will arrive at the satellite during the proper interval of the TDMA frame. In order to accomplish this, a source of common frame referenced timing is required and each station must synchronize its burst transmissions to this common frame reference timing based on satellite range information.
One method of establishing a common frame reference is for one burst in the TDMA frame to be used as the reference. In previous TDMA terminal equipment designs such as disclosed in Contribution of BG/T, System Specification of the INTELSAT Prototype TDMA System, BG-1-18E, Mar. 20, 1974, one earth station is required to act as the reference station and transmit a special reference burst at a fixed position in the TDMA frame. As a result of this rigid arrangement, reference burst replacement, in the event of reference station failure, is a complex process involving the exchange of special messages between a number of stations in the network.
In conventional TDMA systems, such as described in Design Plan for INTELSAT Prototype TDMA Terminal Equipment, presented by Nippon Electric Company, Limited, Tokyo, Japan at the Fourth Digital Satellite Conference in July of 1976, control over the configuration of the burst, the number of bursts in a frame and the corresponding control of the multiplexer and demultiplexer operation is implemented using random hardware logic configurations. This leads to undesirable circuit costs and complexity.
Still further, conventional TDMA terminal equipment has required a separate burst multiplexer/demultiplexer to control the terrestrial interface ports. See, for example, the above-referenced BG/T publication in addition to O. G. Gabbard, "Design of a Satellite Time-Division Multiple Access Burst Synchronization", IEEE Transactions of Communications Technology, Vol. COM-16, number 4, August 1968, pages 589-596; or W. G. Maillet, "Processing of the INTELSAT/IEEE International Conference on Digital Satellite Communications", November 1969, pages 69-80. This use of a separate multiplexer/demultiplexer has resulted in an undesirable increase in the cost and complexity of conventional terminal equipment.
It would be desirable to utilize a microprogrammed controller for TDMA terminal control in order to overcome the above-mentioned difficulties. However, a disadvantage of conventional microprogrammed controllers used in digital computer systems is their lack of accurate resolution in timing control. Conventional systems have utilized microprogrammed controllers such as disclosed by J. L. Nichols, "A Logical Next Step for ROM", Electronics, June 1967, pp. 111-113; K. J. Thurber, R. O. Berg, "Universal Logic Modules Implemented Using LSI Memory Techniques" Fall Joint Computer Conference Proceedings, November 1971, pp. 177-194; or Signetic Corporation, Digital/Linear MOS Applications Handbook, 1974, pp. 24-47. Each of these designs has been primarily intended for use in digital computers where the emphasis is placed on control rate rather than control resolution. Accordingly, such microprogrammed controllers would not operate satisfactorily in a TDMA system.